Environmental sensing systems may include a variety of sensors to determine the presence and concentrations of hazardous (i.e., combustible) and/or toxic gases in industrial and other workspaces. Depending on the environment, it may be desirable to provide sensing information for a wide variety of different gas types and concentrations. Thus, a typical system can include a large number of different types of sensors, where each type of sensor is adept at sensing a particular gas in a desired concentration range.
In order for a particular sensor to detect a desired gas in a particular concentration range, and to transmit that information in a form readily understood by a remote transmitter, the sensor's output signals must be properly conditioned. Currently, sensor signal conditioning is accomplished by using discreet components (i.e. resistors, capacitors, operational amplifiers, etc.) to filter and amplify a specific sensor's output prior to performing a conversion to the digital domain for further processing. In one example, signals from electrochemical sensors are typically conditioned using the well-known potentiostat circuit. The drawback to using discrete components, however, is that the arrangement of such components is often specific to a particular type of sensor, and also to a particular gas being sensed as well as a desired concentration range. Thus, developing conditioning circuits for a wide range of gases and ranges requires changing the values of these components to achieve optimum analogue signal conditioning. This, in turn, requires a wide variety of conditioning circuits to cover ranges of potential interest. As a result, a large number of sensors of differing types, and of differing concentration range capacities, are manufactured and stocked to meet the associated wide variety of field applications. Moreover, most sensors operate in combination with an associated transmitter unit. Due to the specialized nature of the described sensors, such transmitters often only work with a single sensor type. As such, current systems require that a similarly large number of different transmitters are also manufactured and stocked.
In addition, when current remote transmitter and sensors are installed and/or replaced, they are individually adjusted to ensure they are appropriately calibrated, and also to ensure that they are in proper working condition. Currently, for remote transmitter and sensor applications this adjustment/verification process is a two-person effort in which one person stands at the sensor location reading a digital voltage meter, and a second person at the transmitter adjusting a manual potentiometer to achieve a desired output voltage for supplying the sensor. When this operation takes placed in a hazardous area, it can require that the area be declassified so that the transmitter can be opened to access the manual potentiometer. Much the same is true for integral transmitter and sensor applications, in which one person reads a digital voltage meter and adjusts a manual potentiometer at the transmitter to achieve a desired output voltage for supplying the sensor. This can also undesirably involve declassifying the associated hazardous area to open the transmitter to access the manual potentiometer.
It will further be appreciated that sensors undergo sensitivity losses over time. Present systems are not able to provide automatic recognition and adjustment of sensors to compensate for such losses in sensitivity. This, in turn, can lead to premature disposal of sensors that drop below a desired sensitivity threshold. Since such sensors ostensibly would continue to function desirably if their loss in sensitivity could be compensated for, current systems produce unnecessary waste.
Accordingly, there is a need for an improved environmental sensing system that: enables a single transmitter to recognize and accept a plurality of different sensor types, automatically adjusts installed sensors to reduce or eliminate the need for manual adjustment, automatically calibrates sensors to enable a single sensor to accommodate a variety of different sensing ranges, enables a sensor to be calibrated at a single value and then be used at a variety of values, and enables automatic adjustments to extend sensor lifetime.
In addition, a type of environmental sensing system includes a transmitter portion connected to an associated sensor portion by a cable. The transmitter portion transmits information received from the sensor portion to a wireless network, for example. The sensor portion may be located in a hazardous and/or combustible environment remote from the transmitter portion. Further, the transmitter and sensor portions each include a gland arrangement having multiple holes through which wires extend.
It is frequently desirable to “hot swap” the sensor during use, i.e. replace the sensor without declassifying the hazardous area, in the event that the sensor has lost sensitivity, for example. However, removing the sensor may cause generation of a spark or an electrical arc in the connection between the transmitter and sensor circuitry. These sparks could ignite a potentially explosive atmosphere.